This invention relates generally to the fabrication of metal patterns on thin polyimide membranes and more particularly to the production of masks for x-ray lithography.
Soft x-ray lithography (see Soft X-Ray Lithographic Apparatus and Process, Smith et al, U.S. Pat. No. 3,743,842, July 3, 1973) has been shown to be an effective and convenient means of replicating high resolution patterns. This capability has been demonstrated by the replication in an x-ray sensitive resist of a repetitive grating with linewidths less than 160 nanometers. In general, the masks used in x-ray lithography consist of a thin transmitter membrane which acts as a mechanical support for the absorber pattern. High attenuation in the mask membrane leads not only to long exposure time, but also to a reduction in the effective contrast of the mask. This is because the softer x-rays which are desirable for lithography are attenuated more than the harder x-rays. It is therefore desirable to minimize the attenuation of the soft x-rays in the mask transmitter membrane by utilizing a material with low absorptivity and by making the thickness of the transmitter membrane as small as possible.
Several materials have been used to fabricate x-ray transmitter membranes. These include silicon (See "Soft X-Ray Mask Support Substrate", David L. Spears et al, U.S. Pat. No. 3,742,230, June 26, 1973), Al.sub.2 O.sub.3 (See P. A. Sullivan and J. H. McCoy, IEEE Trans. Electron. Devices, ED-22, 412, (1976)), Si.sub.3 N.sub.4 (See E. Spiller et al, Solid State Technol., 19 62, (1976)) and Mylar .RTM. and polyimide (J. S. Greeneich, IEEE Trans. Electron Devices, ED-22 434, (1975)). Silicon membranes thinner than 3 micrometers have not been fabricated. Silicon membranes are also opaque to visible radiation, a fact which makes the use of optical alignment techniques difficult. Al.sub. O.sub.3 membranes as thin as 0.2 micrometers have been produced. However, they are quite fragile and are limited to small areas. Si.sub.3 N.sub.4 and Si.sub.3 N.sub.4 -SiO.sub.2 membranes of 0.1 and 0.2 micrometer thickness, respectively, have been fabricated, but they also are very fragile and have been limited to areas smaller than 3 mm by 3 mm. Mylar.RTM. membranes have been used extensively for x-ray masks. The minimum thickness of Mylar.RTM. films used for x-ray masks has been limited to the commercially available thickness of 3 micrometers. A major difficulty encountered in the use of Mylar.RTM. mask transmitter membranes has been the extreme roughness of the surface of commercially available Mylar.RTM.. In addition, it is necessary to provide a means of cooling the thin, heat sensitive polymer film when depositing metal on it or during ion beam etching of the x-ray absorber pattern. Commercially available polyimide films as thin as 6 micrometers have been used as x-ray transmitter membranes. The surface of commercial polyimide is considerably smoother than the surface of commercial Mylar.RTM.. However, the difficult problem of heat-sinking the membrane during metal deposition and etching of the absorber pattern remains.
It is therefore the object of this invention to provide a novel and useful mask and a process for fabricating an x-ray mask whose transmitter membrane has low x-ray attenuation, has a smooth surface, and is optically transparent. It is a feature of this invention that the thin mask membrane remains attached to a glass substrate during fabrication of the absorber pattern thus eliminating any problems of heating during absorber pattern formation.